1. Field of the Invention
The present invention relates to a semiconductor laser element having a window structure in a vicinity of an end facet.
2. Description of the Related Art
In semiconductor laser elements, currents generated by optical absorption in vicinities of end facets generate heat, i.e., raise the temperature at the end facets. Then, the raised temperature reduces the semiconductor bandgaps at the end facets, and therefore the optical absorption is further enhanced. That is, a vicious cycle is formed, and the end facets are damaged. This is the so-called catastrophic optical mirror damage (COMD). Thus, the maximum optical output power of the semiconductor laser elements is limited by the COMD. The optical output power level (COMD level), at which the COMD occurs, is lowered with, degradation of the end facet and the like caused by aging. Therefore, the COMD occurs in the end, and the semiconductor laser elements may suddenly break down.
In order to suppress the COMD, semiconductor laser elements having a window structure have been proposed, where the window structure suppresses optical absorption at a resonator face. For example, Kazushige Kawasaki et al. (xe2x80x9c0.98 xcexcm band ridge-type window structure semiconductor laser (1),xe2x80x9d Abstracts of the Spring Meeting of the Japan Society of Applied Physics, 1997, 29a-PA-19) disclose a semiconductor laser element which emits laser light in the 980 nm band and has a window structure. The window structure is formed by injecting Si ions into near-edge regions of a ridge structure and disordering an In0.2Ga0.8As quantum well by thermal diffusion. However, the process for producing this semiconductor laser element is very complicated and long since the vicinities of end facets are required to be insulated by injection of H ions in order to prevent a current flow in the vicinities of the end facets.
On the other hand, in order to suppress variations due to aging in the resonator faces of a semiconductor laser element, semiconductor laser elements having processed end facets have been proposed. For example, H. Horie et al. (in xe2x80x9cReliability improvement of 980-nm laser diodes with a new facet passivation process,xe2x80x9d IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5 (1999), No. 3, pp. 832-838) disclose a semiconductor laser element having an internal current confinement structure and emitting laser light in the 980 nm band. The semiconductor laser element comprises an InGaAs active layer, GaAs optical waveguide layers, AlGaAs cladding layers, and an AlGaAs current confinement layer. In addition, cleaved end facets are irradiated with Ar ions having energy not higher than 35 eV, and coated with silicon by evaporation. Then, AR/HR coatings are realized on the end facets by an ion assist evaporation method, where the average acceleration voltage for Ar ions is 110 eV. Thus, this semiconductor laser element can achieve high output power and reliability. Further, Horie et al. report that when the temperature is lowered during growth of the GaAs lower cladding layer, and the InGaAs active layer is grown at low temperature, the quality is improved. However, in order to produce this semiconductor laser element, the low-energy ion acceleration requires expensive equipment.
The object of the present invention is to provide a semiconductor laser element which is reliable in operation with high output power, and can be produced by a relatively simple process without using expensive equipment.
According to the present invention, there is provided a semiconductor laser element having opposite end facets which realize a resonator, comprising: a substrate of GaAs of a first conductive type; a lower cladding layer of the first conductive type, formed above the GaAs substrate; a first optical waveguide layer being made of GaAs of the first conductive type or an undoped type, having a first bandgap, and being formed above the lower cladding layer; an active layer being made of one of InGaAsP and InGaAs, having a compressive strain and a second bandgap smaller than the first bandgap, and being formed above the first optical waveguide layer so as to leave at least one first space in at least one first vicinity of at least one of the opposite end facets; a second optical waveguide layer being made of GaAs of a second conductive type or an undoped type, having a third bandgap greater than the second bandgap, and being formed above the active layer; a third optical waveguide layer being made of GaAs of the second conductive type or an undoped type, having a fourth bandgap greater than the second bandgap, and filling the at least one first space; and an upper cladding portion formed above the second optical waveguide layer.
That is, the semiconductor laser element according to the present invention comprises the GaAs optical waveguide layers, and the active layer does not exist in at least one vicinity of at least one of the opposite end facets, and the at least one space produced in the at least one vicinity of the at least one of the opposite end facets is filled with the GaAs optical waveguide layer. Thus, a so-called window structure is formed.
In addition, it is preferable that the above window structure is formed in vicinities of both of the opposite end facets.
Further, the upper cladding portion may be constituted by either a single layer or multiple layers.
Preferably, the semiconductor laser element according to the present invention may also have one or any possible combination of the following additional features (i) to (xi).
(i) The active layer may be made of Inx3Ga1-x3As1-y3Py3, where 0.49y3 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1. That is, when y3=0, the active layer is made of InGaAs, which does not contain P.
(ii) The semiconductor laser element according to the present invention may further comprise: a contact layer of the second conductive type formed above the upper cladding portion so as to leave at least one second space in at least one second vicinity of the at least one of the opposite end facets; and an electrode formed on the contact layer. The electrode is one of a pair of electrodes which are normally provided in the semiconductor laser element.
(iii) The semiconductor laser element having the feature (ii) may further comprise an insulation film formed in the at least one second space above the upper cladding portion, and the electrode is formed on the insulation film as well as the contact layer.
(iv) In the semiconductor laser element according to the present invention, the upper cladding portion may include a ridge portion which has a stripe form and extends between the opposite end facets, where the top of the ridge portion is higher in elevation than both sides of the ridge portion of the upper cladding portion.
Alternatively, an index-guided structure may be realized by an internal current confinement structure, which is specifically realized as in the following features (v) to (x).
(v) In the semiconductor laser element according to the present invention, the upper cladding portion may comprise: a first etching stop layer made of GaAs of the second conductive type; a second etching stop layer being made of Inx8Ga1-x8P, having a first stripe opening for current injection and being formed above the first etching stop layer, where 0xe2x89xa6x8xe2x89xa61; a current confinement layer being made of Ga1-z2Alz2As of the first conductive type, having a second stripe opening for current injection, and being formed above the second etching stop layer; a cap layer being made of GaAs, having a third stripe opening for current injection, and being formed above the current confinement layer; and a first upper cladding layer of the second conductive type formed above the cap layer. The first, second, and third stripe openings extend between the opposite end facets.
(vi) In the semiconductor laser element according to the present invention, the upper cladding portion may comprise; a first etching stop layer made of Inx8Ga1-x8P of the second conductive type, where 0xe2x89xa6x8xe2x89xa61; a second etching stop layer being made of GaAs, having a first stripe opening for current injection, and being formed above the first etching stop layer; a current confinement layer being made of In0.49(Ga1-z4Alz4)0.51P of the first conductive type, having a second stripe opening for current injection, and being formed above the second etching stop layer, where 0xe2x89xa6z4 less than 1; a cap layer being made of In0.49Ga0.51P of the first conductive type, having a third stripe opening for current injection, and being formed above the current confinement layer; and a first upper cladding layer of the second conductive type formed above the cap layer. The first, second, and third stripe openings extend between the opposite end facets.
(vii) In the semiconductor laser element having the feature (v) or (vi), the lower cladding layer and the first upper cladding layer may be made of one of AlGaAs, InGaAlP, and InGaAlAsP which lattice-matches with the substrate.
(viii) In the semiconductor laser element having the feature (v) or (vi), the upper cladding portion further may comprise a second upper cladding layer of the second conductive type formed under the first etching stop layer.
(ix) In the semiconductor laser element according to the present invention, the upper cladding portion may comprise; a first upper cladding layer being made of In0.49Ga0.51P of the second conductive type; an etching stop layer being made of GaAs, having a first stripe opening for current injection, and being formed above the first upper cladding layer; a current confinement layer being made of In0.49(Ga1-z4Alz4)0.51P of the first conductive type, having a second stripe opening for current injection, and being formed above the etching stop layer, where 0xe2x89xa6z4xe2x89xa61; a cap layer being made of In0.49Ga0.51P, having a third stripe opening for current injection, and being formed above the current confinement layer; and a second upper cladding layer of the second conductive type formed above the cap layer. The first, second, and third stripe openings extend between the opposite end facets.
(x) In the semiconductor laser element having the feature (viii) or (ix), the lower cladding layer, the first upper cladding layer, and the second upper cladding layer may be made of one of AlGaAs, InGaAlP, and InGaAlAsP which lattice-matches with the substrate.
(xi) The semiconductor laser element according to the present invention may further comprise tensile-strain barrier layers respectively formed between the first optical waveguide layer and the active layer and between the active layer and the second optical waveguide layer and made of Inx4Ga1-x4As1-y4Py4, where 0xe2x89xa6x4xe2x89xa60.49y4 and 0xe2x89xa6y4xe2x89xa60.5.
The present invention has the following advantages.
(a) As mentioned before, according to the present invention, the window structure, i.e., a region transparent to oscillated light, is formed in at least one vicinity of at least one of least one of the opposite end facets. Therefore, it is possible to reduce heat generation in the at least one vicinity of at least one of the least one of the opposite end facets, and significantly raise the COMD level. Thus, reliability of the semiconductor laser element in high output power operations can be increased.
(b) Since the optical waveguide layers are made of GaAs, it is possible to lower the temperature during growth of the GaAs lower (first) optical waveguide layer, and grow the active layer at low temperature. Therefore, the quality of the active layer can be improved.
(c) When the contact layer of the second conductive type is formed above the upper cladding portion so as to leave at least one second space in at least one second vicinity of the at least one of the opposite end facets, current injection into the window structure can be suppressed. Therefore, it is possible to further increase optical output power.
(d) When an index-guided structure is realized by forming a ridge structure in the upper cladding portion or an internal current confinement structure, it is possible to accurately control the oscillation mode of laser light.
(e) When the upper cladding portion in the semiconductor laser element according to the present invention has a multilayer structure, the etching depth can be easily controlled by internally arranging etching stop layers made of InGaP-based and GaAs-based materials and utilizing the fact that the InGaP-based and GaAs-based materials are selectively etched. Thus, the index-guided structure can be easily and accurately produced.